As you can notice, the 409 stainless is marked there, however it might not always be visible on your tubing. Sometimes the bend might be where the marking is, or it might be part the cutting the tube. You can also see it's slightly shinier than the Aluminized/Mild Steel.

So overall remember stainless comes in many forms, and the old 'magnet' test isn't always an accurate way. Just think of all those stainless fridges with little billy's pictures being held on by magnets ;)